The present invention is based on a hydraulically operated change-over valve of the type disclosed in German patent application No. 42 36 045, for example. The valve is intended for use in hydraulic brake systems with anti-lock control operating according to the return principle in anti-lock control operations, to which end they are provided with a return pump. To permit the propagation of pressure fluid into the wheel brakes for traction slip control without pedal-operated braking, the return pump is of the self-priming type and includes a suction line to the brake line. A separating valve is inserted into the brake line between the port of the suction line and the port of the pressure line of the return pump. To conduct the pressure fluid into the wheel brakes during pedal-operated braking, rather than simply to the suction side of the return pump, a hydraulically operated change-over valve which is controlled by way of the master cylinder pressure is inserted into the suction line. The master cylinder pressure is delivered into the inlet chamber of the valve and propagates through the valve seat into the outlet chamber. The suction side of the return pump is connected to the outlet chamber. The master cylinder pressure in the outlet chamber acts upon the first diaphragm causing it to move in the direction of the atmosphere. This enables the closure member, which along with its tappet is supported on the diaphragm, to move into abutment with the valve seat and close the change-over valve. From now on, the master cylinder pressure acts in the inlet chamber only. The outlet chamber remains connected to the suction side of the return pump only.
Without pedal operation, the change-over valve remains open because no pressure is applied from the outlet chamber to the diaphragm. In mere pedal-operated normal braking operations or in traction slip control operations, when the brake pedal is not applied, the hydraulically operated change-over valve operates reliably and with low noise. However, when the brake pedal is operated during traction slip control, for example, because the driver has recognized that the desired acceleration is impossible to achieve, or when the brake pedal is released after anti-lock control and the return pump is still running, the valve frequently starts to flutter and produces loud noises. This is due to the fact that the separating valve in the brake line is being opened or is open during pedal-operated braking operations so that the pressure pulses of the return pump propagate until the inlet chamber of the change-over valve as long as the master cylinder pressure is lower than the closing pressure to overcome the compression spring which acts upon the piston arranged on the atmosphere side. Subsequently, the pressure pulses of the return pump act so as to close the change-over valve, while in the outlet chamber the suction phases of the return pump act in the opening sense. Thereby, the valve is possibly pushed until it opens.
An object of the present invention is to provide a valve of the type referred to hereinabove involving low-noise operation in all phases of brake application.